What we saw in the episode is definitely beyond the advanced stage of decomposition, since the advanced stage is the "end of the active decay process". If this corpse was at a stage before advanced, it wouldn't be dry to the point where it's skin has flaked off the bone.

Also remember that this is a writer who wrote a planet's temperature being -291 degrees celsius (That's below absolute zero) and assumed that we would never be able to solve Fermat's last theorem.

I won't argue that the episode's science is iron clad -- it clearly isn't. And you're probably right that the condition of the remains could likely be considered advanced in decomposition. But further decay could still conceivably occur if Richey's corpse is indeed mummified. If the tissue were dessicated due to an extreme drying out, then by simply exposing the remains to moisture the decay process would continue. One thing's for sure: the remains are clearly not simply skeletal.

Now, I'm not sure I agree with your suggestion that Richey should have been depicted as a normal person resting peacefully. More likely, he should have maybe looked more like Ítzi the Iceman, which could be explained by the earlier dialogue (I know it's incorrect) of extreme coldness on the planet. I think perfect preservation would be highly unlikely though.

But on this we can both agree, Data's dialogue needed to be tweaked to closer match what we see on screen because it doesn't really matter that the environment is sterile -- Richey would've had live bacteria in his gut. Either my suggestion where Data adds that the environment in the building is (or was before they got there) extremely dry... or stating explicitly that Richey's been mummified. Maybe something like this:

DATA
He has been dead for two hundred
eighty-three years. The partial
mummification is due to the unusually
dry environment.

__________________"Shake off all the fears & servile prejudices, under which weak minds are servilely crouched. Fix reason firmly in her seat, and call to her tribunal every fact, every opinion." -Thomas Jefferson